This invention relates to apparatus for crushing granular substances using a traveling magnetic field.
Heretofore, ball mills, vibration mills, jet mills and the like have been employed for crushing granular materials. The known ball mill crushers comprise a cylinder which is turned on its horizontal axis and which house, e.g., steel balls which, when the cylinder is turned about its axis, move upwardly in the cylinder and thereafter tumble down onto the base of the cylinder. Strong shock motion and frictional action are thus applied to the material to be crushed when the steel balls are falling, forcing the granular material against the inside wall of the cylinder and, thereby, crushing it. However, when the steel balls are falling, energy is dissipated due to the collision of the steel balls with each other. Thus, the total energy available is not utilized for crushing purposes. Consequently, the power employed to lift the steel balls is not effectively utilized. In addition, the ball mill has a counterbalancing disadvantage in allowing the motive power to be consumed by the energy loss incurred during the transmission of a driving force from a driving apparatus through a driving gear to turn the cylinder, as well as loss in the bearings when the cylinder is made to turn.
In known vibration mills, energy is wasted by the steel balls colliding with each other as in a ball mill. Further disadvantages include noise and vibration caused by vibrating a container containing granular materials in tens of cycles.
With respect to the known jet mills, (also known as fluid energy mills) these are designed to accelerate fine grains of granular material, by means of a jet stream obtained by jetting air or heated steam (pressurized up to several times atmospheric pressure) from a nozzle several millimeters in diameter, so that the fine grains are crushed when they collide with each other and with the interior walls of the device. However, these jet mills are unable to crush grains several millimeters in diameter down to pieces on the order of a micron, and are mainly employed to crush extra fine grains less than a few microns across (the crushing efficiency being extremely worsened when used for crushing larger diameter grains). Grain diameters suitable for crushing in the case of the jet mill are normally less than 500 .mu.m, which disadvantageously requires preliminary crushing. Another disadvantage is that its crushing capability is limited to several tens of microns even if particles of 500 .mu.m in diameter may have been crushed. Still another disadvantage is the necessity of re-crushing the above grains to pieces less than 1 .mu.m in diameter.
In place of these above-described known crushers, which result in energy loss and necessitate a long period of time when crushing granular materials, an apparatus used to crush granular materials by the utilization of a magnetic field, shown in FIG. 1, is known from Japanese Patent Announcement No. 51-5991. Referring to FIG. 1, there is shown a reactor 1 enclosing pieces of ferromagnetic material 2, a guide bush 3 for the reactor 1, a rotating magnetic field generator 4, and an assembly 5 comprising a crank and a connecting rod. In this known apparatus, the assembly 5 is turned in the direction of the arrow to impart an alternating motion to the reactor 1, which contains the mixture of granular materials and the pieces of ferromagnetic material 2, against the rotating magnetic field generator 4. The pieces of ferromagnetic material 2 contained in the reactor 1 rotate in a direction parallel to the rotating magnetic field produced by the rotating magnetic field generator 4. Thus, the pieces of ferromagnetic material 2 start rotating rapidly, causing the granular materials to be crushed.
Because the pieces of ferromagnetic material 2 are attracted by the rotating magnetic field and stay within the zone affected by the field despite the alternating motion of the reactor 1, the granular material contained in the reactor 1, which is longer than the rotating magnetic field generator 4, can be crushed, thus making it possible to process a larger amount of the granular material.
However, the strength of the magnetic field is weakened as its distance from the guide bush 3 increases. The crushing strength also is reduced because the motion of the pieces of ferromagnetic material 2 gradually slows. Therefore, it is impossible to construct and operate a large-sized apparatus of this type, the reason being that the diameter of a reactor 1 for such an apparatus would have to be relatively large, which in turn would result in a weak magnetic field in the central portion of the reactor. As a result, grains are not effectively crushed in places other than the area close to the rotating magnetic field generator 4, or the portion close to the internal circumferential surface of the reactor 1. Thus, some grains may be left uncrushed. Furthermore, the rotating magnetic field is unidirectional, and renders uniform the movement of the pieces of ferromagnetic material. In this case, the disadvantage is that the movement of the pieces of ferromagnetic material is not sufficiently effective to obtain a satisfactory crushing effect.